Refrigerators have become indispensable for the preservation of temperature-sensitive items such as food, drink, medicines, pharmaceuticals, vaccines and the like.
As defined herein, the term "refrigerator" includes refrigerators capable of cooling to temperatures below 4.degree. C., and/or freezers which cool to -20.degree. C. or below.
In simplest form, a refrigerator comprises a compressor in fluid communication with a coolant circuit (which includes a coolant such as R134a), the coolant circuit being in thermal communication with the interior of a cooling chamber, in which are stored items to be cooled or frozen.
The principle of operation is that the compressor pressurizes the coolant which circulates through the coolant circuit located in the walls of the cooling chamber, whereupon the coolant expands drawing heat from the cooling chamber and contents therein, resulting in cooling or freezing of the cooling chamber and contents.
The expanded coolant is then re-compressed and recirculated through the circuit. Usually, a thermostat is provided which assists maintenance of a desired temperature in the cooling chamber.
Typical refrigerator compressors are provided with induction motors which operate at a fixed speed with a fixed energy requirement, and as such are relatively wasteful in terms of energy consumption.
Recently, variable-speed compressors have been developed which can operate at variable speeds and hence with variable energy consumption, thereby providing enhanced energy efficiency. It will be appreciated that variable-speed compressor technology has progressed rapidly in recent years, such as may be found in International Application WO98/15790, Japanese Patent No. 10038439, Japanese Patent No. 9196535 and U.S. Pat. No. 5,711,159, for example. Of particular note are the TLV alternating current (AC) compressor and the BD35F and BD50F direct current (DC) compressors produced by Danfoss A/S.
Although in many cases refrigerators operate via mains powered AC electricity, an increasingly used alternative source of energy for refrigerators is solar energy. Usually, solar energy operation is achieved using solar panels which collect and convert incident solar radiation into electrical energy which is storable in batteries. When incident solar radiation is minimal, such as at night, early morning or late evening, or in overcast conditions, the batteries can be used to power the refrigerator compressor.
A disadvantage of this system is that typical refrigerator compressors require a threshold electrical current, often 5 Amps, to be operable. Thus, the refrigerator is only operable once the threshold current necessary to power the compressor has been reached.
A further disadvantage with such systems is that the current regulators between the solar panels and batteries are unreliable. If they fail, the batteries will either not be charged or be damaged by uncontrolled current input.
Also, existing solar-powered refrigerators are often not readily transportable. This creates a particular problem in remote regions where solar powered refrigeration is often used to prevent spoilage of medicines and vaccines. For example, it is common for vaccines to be transported from central storage locations to surrounding regions via couriers carrying vaccine containers equipped with freezer blocks A ready supply of these blocks must be maintained.
However, in conditions of low sunlight, it can be extremely difficult to maintain this supply as the typical solar-powered refrigerators used for this purpose become inoperable due to their requirement for a threshold current to be provided. Although batteries can provide power under conditions of low sunlight, they typically have the shortest lifespan of any of the components of the refrigeration system. Consequently, they tend to constitute the weakest "link" in the system. Furthermore, batteries can be stolen or otherwise become inoperable, and therefore require replacement. The high replacement costs and lack of ready availability of solar-chargeable batteries pose a particular problem in remote areas of poor countries.